Most automobiles are now provided with a pneumatic booster. Pneumatic booster utilizes a pressure difference between a negative pressure occuring in an air intake system of an engine of an automobile and the atmospheric pressure so as to enhance the force generated by depressing a brake pedal and thereby generating a much greater braking force in response to a minimal force applied to a brake pedal.
One example of such a pneumatic booster is shown in FIG. 1.
Referring now to FIG. 1, a housing 3 includes a front shell 1 and rear shell 2. A center plate 4 positioned in the housing 3 divides the inside of the housing 3 into two sections, i.e., front and rear chambers. A front constant pressure chamber 7 and a front variable pressure chamber 8 are defined within the front chamber by means of a front power piston 6 having a diaphragm 5 thereon. A rear constant pressure chamber 11 and a rear variable pressure chamber 12 are also formed in the rear chamber by means of a rear power piston 10 having a diaphragm 9 thereon. The front shell 1 is provided with a negative pressure conduit 13 for connecting the front constant pressure chamber 7 with a negative pressure source, such as an intake manifold (not shown).
The front power piston 6 and rear power piston 10 are secured at their respective inner periphery around the surface of a valve body 15 of a generally cylindrical configuration. The valve body 15 is formed with a recess 14 at the front portion thereof. A return spring 16 is interposed between the portion of the recess 14 of the valve body 15 and the front shell 1 so as to normally urge backward the front power piston 6 and rear power piston 10 through the valve body 15. an output rod 18 is fixedly received within the recess 14, with a reaction disk 17 disposed therebetween. A plunger 19 is reciprocably inserted in an internal bore 21 of the valve body 15 with the fron end thereof being close to the reaction disk 17. The plunger 19 in turn receives the forward end of an input rod 20 in a recess formed in the rearward end thereof. It will be noted that a brake pedal (not shown) is connected to the input rod 20.
The valve body 15 includes a first passage 22 communicating the front variable pressure chamber 8 with the rear variable chamber 12, a second passage 23 communicating the front constant pressure chamber 7 with the rear constant pressure chamber 11, a third passage 24 communicating the rear variable pressure chamber 12 with the internal bore 21, and a fourth passage 25 for communicating the rear constant pressure chamber 11 with the internal bore 21. The valve body 15 also houses therein a poppet valve 26. The poppet valve 26 comprises a valve member 27 attached to the inner periphery of the internal bore 21 at a position rearward of the fourth passage 25, a first valve seat 28 formed around the inner periphery of the internal bore 21 at a position forward of an opening of the fourth passage 25 to the internal bore 21, and a second valve seat 29 formed in the rearward end of the plunger 19. The valve member 27 is so positioned as to be spaced from the first valve seat 28 by the distance of S.sub.1 when in an initial state, i.e., when the brake pedal has not yet been depressed.
The third passage 24 also serves as a bore 30 for receiving therein a stop key 32 which will be explained below. The plunger 19 is formed with a groove 31 having a predetermined axial width at a position corresponding to the bore 30. The above-described stop key 32 is freely inserted into the bore 30 and groove 31, the stop key functioning to limit the position of the plunger 19 in the initial state.
With the construction of the pneumatic booster having been described just above, the valve member 27 in the inital states is seated on the second valve seat 29, while rising from the first valve seat 28 as described above. Accordingly and in the inital state, the front constant pressure chamber 7, rear constant pressure chamber 11, front variable pressure chamber 8 and rear variable pressure chamber 12 all are in communication with one another and under a negative pressure.
With the above condition and when a brake pedal is stepped down, the input rod 20 starts to move forwardly. When the input rod 20 has travelled a predetermined stroke or distance which generally corresponds to distance S.sub.1 mentioned above, the valve member 27 becomes to be seated on the first valve seat 28 and to rise from the second valve seat 29. Accordingly, atmospheric air is introduced into the rear variable pressure chamber 12 and the front variable pressure chamber 8 through the third passage 24 and the first passage 22. Thus, a pressure difference is caused between the front and rear variable pressure chambers 8 and 12, respectively, into which air at atmospheric pressure has been introduced, and the front and rear constant pressure chambers 7 and 11, respectively, which are under a negative pressure. By virtue of this pressure difference, the valve body 15 and output rod 18 are forwardly driven with substantial force by means of the front power piston 6 and the rear power piston 10.
It is to be understood that a brake mechanism having a shorter pedal travel, i.e., displacement of a brake pedal during an initial stage of braking operation, is desirable. With the construction of the above prior art pneumatic booster, however, a booster action will be exerted only after the output rod 18 has travelled a predetermined stroke and thereafter the atmospheric pressure is introduced into the front and rear variable pressure chambers 8 and 12, respectively. Thus, the prior art pneumatic booster has a drawback in that a relatively long pedal travel is required and hence it does not substantially satisfy the above requirement. In this connection, it is noted that an attempt to create an immediate booster action has been made. Accoding to this attempt, a dimension of the width of each of the stop key 32 and the bore 30 is appropiately selected so that the valve member 27 will be able to be seated on both the first valve seat 28 and second valve seat 29 when in the above initial state. With this construction, the valve member 27 rises from the second valve seat 29 so as to introduce atmospheric air into the front variable pressure chamber 8 and the rear variable pressure chamber 12, immediately upon commencement of a forward movement of the input rod 20, thereby causing a simultaneous booster action. It should be noted however that, in the above construction, the amount of displacement for which the output rod 18 has travelled during the initial braking operation corresponds at most to the displacement of the input rod 20. Thus, above attempt is also insufficient in terms of significantly reducing the amount of pedal travel.